The invention relates to an improved jet nozzle and submerged aeration system for efficiently mixing gas with waste water.
Industrial waste, domestic sewage and the like are commonly purified by pumping the liquid into a large pond, tank or basin where a bacteria population consumes the organic material. Because the dissolved oxygen in the waste water is usually insufficient to support the required population of bacteria, the water must be aerated. This can be done with mechanical surface aerators which have agitators extending into the waste water from above the water surface to agitate the water and incorporate air. Alternatively, air can be diffused through the bottom of the basin, for example, through a porous medium. Surface aerators are not efficient and cause certain mechanical problems. The energy loss of diffusing air is also great and a conventional diffused system is not suitable for installation in an existing pond, because the pond would have to be drained and taken out of service for an extended period.
The waste water can also be aerated by pumping the waste water (mixed liquor) through submerged tubes with openings through which air or other gas is drawn or pumped into the tubes to create turbulent mixing and mass transfer. This type of system is termed jet aeration and is generally more energy efficient than diffusion or surface aeration. The patent to Powers U.S. Pat. No. 2,479,403 describes an early jet aeration system.
The present invention relates to an improved system for mixing a gas such as oxygen or air with waste water in a body of waste water. In a prior art system, a plurality of vortex mixing chambers or jet nozzles is located below the surface of the waste water and water in the body is pumped through each nozzle from an inlet to an outlet. A suitable gas, such as oxygen or air containing oxygen, is injected into each of the vortex mixing chambers at a step region to form parallel streams of air and water in an extending chamber. As the two streams move down the extending chamber, the interface between the two streams becomes unstable and water waves form which trap gas. As the waves grow in amplitude they eventually attach to the sides of the extending chamber. This causes large frictional stresses and creates tiny bubbles which mix with the water. The small bubble-water mixture is then discharged below the surface of the waste water body. The small bubble mixture has a large surface area which transfers oxygen to the waste water.
Since the water and air essentially flow in the same direction, horizontal momentum is exchanged between the gas and water. Inasmuch as there is very little friction between the parallel air and water streams at their interface, virtually no energy is wasted in unnecessary turbulence. At the point of water wave attachment to the extending chamber, frictional and turbulent energies of the vortex formed are harnessed to disperse the gas into a distribution of extremely fine bubbles. Further, the small bubbles are dispersed throughout the water phase resulting in a fine bubble air-water mixture. The vortexing point (the point of wave attachment to the extending chamber) is a function of the gas flow rate. As the flow rate increases, the vortexing point moves downstream. Systems of this type are described and claimed in copending application Ser. No. 598,871, filed July 24, 1975.
In an improved prior art device, the extending chamber is inwardly tapered in the downstream direction to ensure that the vortices created do not extend out of the chamber at high air flow rates which would reduce the efficiency of mixing and mass transfer. Thus, the device operates at a high and constant efficiency over a wide range of air flow rates. Further, helical vanes can be provided in the injection passages for the gas to create greater wave generating conditions which extend the operating range of the device to greater air flow rates. This improved taper system is described and claimed in copending application Ser. No. 863,588, filed Dec. 22, 1977, now U.S. Pat. No. 4,157,304, and co-pending application Ser. No. 953,212, filed Oct. 20, 1978.
As described in the above-mentioned U.S. Pat. No. 4,157,304, in this improved prior art device, the extending chamber is divided into a first section or stage extending from the step region and a second inwardly tapered section or stage extending downstream from the first section. This second section is shorter than the first and tapered at a rate greater than any taper of the first section. It is desired that both sections be tapered, but if only the second section is tapered, the results are satisfactory, and the cost of manufacture is less when machining techniques rather than molding techniques are used. However, when molding in plastic, tapering of the first section aids in removal from the mold. The first section is preferably non-diverging, and the length of the first section along the flow direction is preferably between one and ten times the diameter at the step region. The length of the second section along the flow direction is preferably between one-eighth and one times the diameter at the first and second intersection of the sections, and the taper of the second section is preferably between 11.degree. and 22.degree..
This system can be quickly and easily installed in any existing aeration pond, lagoon, or tank without the need for the facility to be shut down for an extended period and without the need for the pond or tank to be drained--a project which is difficult or impossible to accomplish in most instances. The system can, in fact, be installed and operating within a few minutes. In comparison with diffused air type devices and surface aeration systems, the energy required to incorporate a given amount of oxygen into the water is much less. Further, the bubbles which are produced are tiny, thus creating a good environment for effective use of oxygen by the bacteria within the pond or basin. Many of the other disadvantages of surface aerators and diffusion type devices are also avoided.